As I understand it, the Adobe installer checks to see if you have any 32-bit plugins installed, and if you do it then installs 32-bit Acrobat. As you will not have a 64-bit plugin in a 32-bit Acrobat, the 64-bit version will never install this way. So to move to 64-bit, you need to either remove the 32-bit plugins, and update. Or un-install Acrobat 32-bit completely, and then clean install Acrobat 64-bit, and then the 64-bit plugins.
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